Field of the Invention
The invention relates to a method for acquiring a magnetic resonance image data set of a scan area of an examination subject to be scanned with a magnetic resonance device having a transmitter coil apparatus to emit a radio-frequency signal, in particular with at least two transmission channels, such that different polarizations of the radio-frequency signal can be realized, it being the case that a magnetic resonance sequence is used to capture raw data for the magnetic resonance image data set. The invention additionally relates to a magnetic resonance apparatus designed to implement such a method.
Description of the Prior Art
Magnetic resonance imaging and its principles are widely known. An object to be examined is placed in a static magnetic field with a relatively high field strength. This field is referred to as the B0 field. In order to be able to acquire magnetic resonance data, for example in a slice of the object, the nuclear spins of this slice are excited and the decay of this excitation, for example, is analyzed as a signal. Gradient fields can be generated using a gradient coil apparatus, while a radio-frequency transmitter coil apparatus emits radio-frequency excitation pulses that are often referred to as radio-frequency pulses. The cumulative effect of the radio-frequency pulses (“excitation”) generates a radio-frequency field that is usually referred to as the B1 field and flips (deflects) the spins of resonant excited nuclei, selectively located by the gradients, with respect to the magnetic field lines of the static magnetic field, by an amount known as a flip angle. The excited spins of the nuclei then radiate radio-frequency signals that can be received using suitable receiving antennas, in particular including the transmitter coil apparatus itself, which then also acts as a receiver coil apparatus, and processed in such a way that magnetic resonance image data can be reconstructed.
Conventional transmitter coil apparatuses are operated in a manner referred to as a “homogeneous mode”, for example in a “CP mode” (circular polarization mode), with a single radio-frequency pulse having specific amplitude characteristics and phase differences being applied to all components of the transmitter coil, for example all transmission rods of a birdcage antenna. It has been proposed, in order to increase flexibility and create new scope to improve imaging, also to enable operation known as parallel transmission (pTX), in which multiple transmission channels of a transmitter coil apparatus each have applied to them separate pulses that can differ from one another. For example, transmitter coil apparatuses having two transmission channels have been proposed with which it is possible to realize an elliptical polarization (EP) of the B1 field as well as a circular polarization (CP) by selecting the phase difference between the channels and/or the amplitude characteristics accordingly. Consequently the polarization of a resulting B1 field can be described for each of the at least two channels by means of the parameters phase and amplitude, for example. Such a multi-channel pulse (excitation), which is composed of separate pulses for the different transmission channels, is often referred to as a “pTX pulse” (for “parallel transmission”). It should be noted that it is not essential when using two transmission channels that these channels be operated independently, because different polarizations can also be realized by a single actuation channel with a single amplifier device. The pulse shape is then the same for both transmission channels=elements of the transmitter coil; the only difference is the phase and/or amplitude. The following is also intended to encompass apparatuses of this type where at least two transmission channels are concerned.
Inhomogeneities of the B1 field specifically also have an effect on the quality of the magnetic resonance image data in the case of measurements with a high magnetic field strength of the static magnetic field, for example with field strengths of greater than or equal to 3 tesla. It is problematic in this connection that the nature of the inhomogeneities varies as a function of the electrical and dielectric properties of the examination subject, in particular a patient, which makes it difficult to realize a generally applicable correction. The flip angle actually achieved thus also exhibits local variation and no longer corresponds to the desired value throughout the scan area. The principal effects of such B1 field inhomogeneities in the magnetic resonance image data set are fluctuations in the image brightness and contrast. Variations in the B1 field strength, irrespective of whether in the positive or negative direction, can in particular lead to a marked localized loss of intensity, even to the extent of complete signal dropout, in some magnetic resonance sequences. This can make it more difficult, even impossible, to assess the magnetic resonance image data in such areas, especially for diagnostic purposes.
Solutions to resolve the problems caused by B1-field inhomogeneities have been proposed. It is evident that the nature of the inhomogeneities is affected by the polarization of the B1 field. As noted above, transmitter coil apparatuses have been proposed that have two supply ports, that is to say two transmission channels. If the associated transmitter coils are arranged perpendicular to each other, the result with a phase difference of 90° between the channels and the same amplitude is a circular polarization (CP) of the B1 field, which can represent an optimum with no examination subject. It has been found, however, that CP does not necessarily result in the best homogeneity of excitation, that is to say of the B1 field, when used with an examination subject. The scope afforded by the transmission channels also permits elliptical polarization. It has been shown, in an article by J. Nistler et al., “Homogeneity Improvement Using a 2 Port Birdcage Coil”, Proc. ISMRM 15 (2007), Page 1063, how B1 homogeneity can be improved in the examination subject by means of excitation different to CP (corresponding to elliptical polarization (EP)). This involves varying the amplitudes of the supply voltages and the phase difference between the channels. It is also possible to offset field inhomogeneities as well as generating selectively positioned excitations when working with greater numbers of channels.
A first known approach to realizing an improvement is to select a polarization that has been shown to result in a relatively homogeneous B1 distribution on average across a majority of examination subjects (patients). There may for example be such a polarization, obtained from test measurements, available in each case for different applications/examination areas.
Another known approach provides for the use of what is referred to as patient-adaptive B1 shimming. This involves optimizing the amplitude characteristics and the phase difference of the transmission channel voltages for the examination subject to be scanned in each case. This realization requires that the B1 field distribution be measured prior to each examination (B1 mapping) in order that the optimal transmission parameters can be calculated. The measurement of the B1 field distribution and the calculation of the optimal excitation here require additional measurement time, which becomes even greater the more transmission channels there are. This application is more technically complex as well, because flexible actuation of the available transmission channels must be realized.